/*=========================================================================
 *
 *  Copyright Insight Software Consortium
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *         http://www.apache.org/licenses/LICENSE-2.0.txt
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 *
 *=========================================================================*/
#ifndef __itkCurvatureNDAnisotropicDiffusionFunction_hxx
#define __itkCurvatureNDAnisotropicDiffusionFunction_hxx

#include "itkCurvatureNDAnisotropicDiffusionFunction.h"

namespace itk
{
template< class TImage >
double CurvatureNDAnisotropicDiffusionFunction< TImage >
::m_MIN_NORM = 1.0e-10;

template< class TImage >
CurvatureNDAnisotropicDiffusionFunction< TImage >
::CurvatureNDAnisotropicDiffusionFunction()
{
  unsigned int i, j;
  RadiusType   r;

  for ( i = 0; i < ImageDimension; ++i )
    {
    r[i] = 1;
    }
  this->SetRadius(r);

  // Dummy neighborhood used to set up the slices.
  Neighborhood< PixelType, ImageDimension > it;
  it.SetRadius(r);

  // Slice the neighborhood
  m_Center =  it.Size() / 2;

  for ( i = 0; i < ImageDimension; ++i )
    {
    m_Stride[i] = it.GetStride(i);
    x_slice[i]  = std::slice(m_Center - m_Stride[i], 3, m_Stride[i]);
    }

  for ( i = 0; i < ImageDimension; ++i )
    {
    for ( j = 0; j < ImageDimension; ++j )
      {
      // For taking derivatives in the i direction that are offset one
      // pixel in the j direction.
      xa_slice[i][j] =
        std::slice( ( m_Center + m_Stride[j] ) - m_Stride[i], 3, m_Stride[i] );
      xd_slice[i][j] =
        std::slice( ( m_Center - m_Stride[j] ) - m_Stride[i], 3, m_Stride[i] );
      }
    }

  // Allocate the derivative operator.
  dx_op.SetDirection(0);  // Not relevant, will be applied in a slice-based
                          // fashion.

  dx_op.SetOrder(1);
  dx_op.CreateDirectional();
}

template< class TImage >
typename CurvatureNDAnisotropicDiffusionFunction< TImage >::PixelType
CurvatureNDAnisotropicDiffusionFunction< TImage >
::ComputeUpdate( const NeighborhoodType & it, void *itkNotUsed(globalData),
                 const FloatOffsetType & itkNotUsed(offset) )
{
  unsigned int i, j;
  double       speed, dx_forward_Cn, dx_backward_Cn, propagation_gradient;
  double       grad_mag_sq, grad_mag_sq_d, grad_mag, grad_mag_d;
  double       Cx, Cxd;
  double       dx_forward[ImageDimension];
  double       dx_backward[ImageDimension];
  double       dx[ImageDimension];
  double       dx_aug;
  double       dx_dim;

  // Calculate the partial derivatives for each dimension
  for ( i = 0; i < ImageDimension; i++ )
    {
    // ``Half'' derivatives
    dx_forward[i] = it.GetPixel(m_Center + m_Stride[i])
                    - it.GetPixel(m_Center);
    dx_forward[i] *= this->m_ScaleCoefficients[i];
    dx_backward[i] = it.GetPixel(m_Center)
                     - it.GetPixel(m_Center - m_Stride[i]);
    dx_backward[i] *= this->m_ScaleCoefficients[i];

    // Centralized differences
    dx[i] = m_InnerProduct(x_slice[i], it, dx_op);
    dx[i] *= this->m_ScaleCoefficients[i];
    }

  speed = 0.0;
  for ( i = 0; i < ImageDimension; i++ )
    {
    // Gradient magnitude approximations
    grad_mag_sq   = dx_forward[i]  * dx_forward[i];
    grad_mag_sq_d = dx_backward[i] * dx_backward[i];
    for ( j = 0; j < ImageDimension; j++ )
      {
      if ( j != i )
        {
        dx_aug = m_InnerProduct(xa_slice[j][i], it, dx_op);
        dx_aug *= this->m_ScaleCoefficients[j];
        dx_dim = m_InnerProduct(xd_slice[j][i], it, dx_op);
        dx_dim *= this->m_ScaleCoefficients[j];
        grad_mag_sq += 0.25f * ( dx[j] + dx_aug ) * ( dx[j] + dx_aug );
        grad_mag_sq_d += 0.25f * ( dx[j] + dx_dim ) * ( dx[j] + dx_dim );
        }
      }
    grad_mag = vcl_sqrt(m_MIN_NORM + grad_mag_sq);
    grad_mag_d = vcl_sqrt(m_MIN_NORM + grad_mag_sq_d);

    // Conductance Terms
    if ( m_K == 0.0 )
      {
      Cx = 0.0;
      Cxd = 0.0;
      }
    else
      {
      Cx  = vcl_exp(grad_mag_sq   / m_K);
      Cxd = vcl_exp(grad_mag_sq_d / m_K);
      }
    // First order normalized finite-difference conductance products
    dx_forward_Cn  = ( dx_forward[i]  / grad_mag ) * Cx;
    dx_backward_Cn = ( dx_backward[i] / grad_mag_d ) * Cxd;

    // Second order conductance-modified curvature
    speed += ( dx_forward_Cn - dx_backward_Cn );
    }
  // ``Upwind'' gradient magnitude term
  propagation_gradient = 0.0;
  if ( speed > 0 )
    {
    for ( i = 0; i < ImageDimension; i++ )
      {
      propagation_gradient +=
        vnl_math_sqr( vnl_math_min(dx_backward[i], 0.0) )
        + vnl_math_sqr( vnl_math_max(dx_forward[i],  0.0) );
      }
    }
  else
    {
    for ( i = 0; i < ImageDimension; i++ )
      {
      propagation_gradient +=
        vnl_math_sqr( vnl_math_max(dx_backward[i], 0.0) )
        + vnl_math_sqr( vnl_math_min(dx_forward[i],  0.0) );
      }
    }
  return static_cast< PixelType >( vcl_sqrt(propagation_gradient) * speed );
}
} // end namespace itk

#endif
